Ultrasonic backscatter and collagen in normal ventricular myocardium

Abstract

Integrated ultrasonic backscatter has been related to collagen deposition in fibrotic myocardium. The purpose of our study was to measure the integrated ultrasonic backscatter in the right and left ventricles of 10 normal freshly excised canine hearts and five normal formalin-fixed human hearts. A 2.25 MHz, 50% fractional bandwidth transducer was positioned at the transducer focal distance from the epicardium. The radio frequency backscatter signal, excluding specular reflections, was digitized, squared, and integrated to yield the integrated ultrasonic backscatter (in decibels down from a 100% reflector). The segment of myocardium corresponding to the integrated ultrasonic backscatter sample volume was excised and assayed for hydroxyproline, a marker for collagen. A second purpose of our study was to evaluate the influence of fixation with formalin on the backscatter. Regional integrated ultrasonic backscatter was therefore measured in 10 freshly excised canine left ventricles, which were fixed in 10% formalin for 2 weeks. Integrated ultrasonic backscatter measurements were then repeated. In freshly excised canine hearts, the integrated ultrasonic backscatter from right ventricle was higher than that from left ventricle (-60.4 ± 1.6 [SEM] vs -66.9 ± 1.0 dB; p < .005). Similar results were obtained in human hearts. There were no correlations between integrated ultrasonic backscatter and collagen content (r = .28 and .32 for dogs and humans, respectively). The integrated ultrasonic backscatter in freshly excised canine left ventricles was -64.8 ± 0.7 dB, which was not significantly different than the value of -65.6 ± 0.7 dB measured after fixation with formalin. We conclude that both integrated ultrasonic backscatter levels and collagen content are higher in the right ventricle than in the left ventricle of normal hearts. Formalin is a suitable fixative for ultrasound studies of the myocardium in vitro.